This invention relates to the fabrication of metallic sandwich and integrally stiffened structures, and is particularly directed to a method of making such structures by superplastic forming and diffusion bonding (SPF/DB), employing an improved composition specifically for facilitating breakthrough prior to superplastic forming.
A number of alloys exhibit superplasticity and are capable of being subjected to superplastic forming to produce parts of predetermined shapes. Superplasticity is the capability of a material to develop unusually high tensile elongation with reduced tendency toward local necking during deformation. However, this invention is particularly concerned with superplastic metals which are subject to contamination of surface integrity at forming temperatures. These are termed "reactive" metals. This includes alloys of titanium, zirconium, and the refractory metals.
Diffusion bonding refers to the solid-state, metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to effect intimate surface contact and cause co-mingling of atoms at the joint interface.
U.S. Pat. No. 3,927,817 discloses a method for fabrication of structures in which metal blanks, preferably of a titanium alloy, are joined at selected areas by diffusion bonding at elevated temperatures and pressures, and then subjected to superplastic forming to form a desired structure. The metal blanks are first treated at selected areas with a stopoff material, such as yttria, boron nitride, graphite, or alumina, to prevent bonding at such treated areas during diffusion bonding. During superplastic forming the metal blanks are expanded at the treated (unbonded) areas into contact with shaping members by increasing the internal pressure, preferably with inert gas, thus forming an expanded structure of a desired shape, essentially in a single operation.
Thus, after the bonds between adjacent metal blanks are formed during diffusion bonding, inert gas pressure, such as argon or helium, is applied to the interior network to superplastically form the unbonded portions of the adjacent metal sheets. For such superplastic forming to occur, gas must penetrate the entire interior network of unbonded (stopped off) areas. Initial flow of gas into an inlet, through the unbonded network and out the exit plumbing, is termed "breakthrough." If no breakthrough takes place, or if insufficient breakthrough takes place, acceptable superplastic forming cannot occur. Breakthrough may be characterized by the product of the time and presure required for it to occur. If a combination of high pressure and extended time is required to effect breakthrough, the parts may be ruptured or improperly formed, and scrapped.
Resistance to breakthrough results from the combined effects of: (1) the small cross-sectional area of the stopoff path; (2) the tortuosity and length of the stopoff path; (3) the low gas permeability of the stopoff path after bonding; and, (4) the resistance to overall bending of the unbonded, unsupported span of the sheet metal adjacent to the stopoff and to the tool cavity. Both the small cross-sectional area and the stopoff path length and tortuosity are fixed by design considerations and are not good candidates for process control. However, the permeability and the resistance to bending are both subject to control as hereafter described.
During diffusion bonding, the stopoff layer is hot pressed by the bonding pressure and temperature and its permeability is significantly reduced. Any measures which tend to resist the reduction of permeability will result in an improved stopoff system. If stopoff were not present, the resistance of the unsupported span to bending is simply determined by the material's strength properties and the geometric configuration of the span. For the small deflections necessary to establish the very modest gas flows required for SPF/DB, the bending force, and therefore the breakthrough pressure time requirement, would be quite small except for the narrowest of spans. However, an appreciable additional resistance to bending must be overcome which results from adhesive bonds of the metal sheet to the stopoff compound and cohesive bonds within the stopoff compound. Both of these bond types are created by compression and sintering of the stopoff mass during the sustained pressure and temperature conditions of the bonding cycle. Any attempt to alter the adhesive bonding characteristics of the stopoff to the reactive metal sheet would tend to involve foreign additions and is likely to compromise the inert nature of the stopoff with respect to the sheet. However, cohesive bonding can be influenced by other than chemical changes. Hence it is necessary that the stopoff compound be one which does not develop a high cohesive strength nor very low permeability during diffusion bonding and thereby resist breakthrough prior to the superplastic forming operation.
In addition, it is necessary that the stopoff compound or composition be inert with respect to the reactive metal surface, so as to cause no formation of alpha-phase case on the metal surface. Alpha-case is the phenomenon whereby a reactive metal such as a titanium alloy that displays a normal microstructure, consisting of a mixture of alpha-phase (hexagonal crystal structure) grains and beta-phase (body centered crystal structure) grains, is converted to all alpha at and near the exposed surface by the diffusion of oxygen and/or nitrogen into the alloy matrix. The conversion to alpha structure, which has a high interstitial (oxygen and/or nitrogen) content, leads to a brittle layer on the surface which is particularly undesirable under fatigue loading conditions because of a tendency to initiate fatigue cracks.
Of a variety of stopoff compounds investigated, including alumina, graphite, boron nitride, silicon nitride, and others, yttria was the only one which was found to be sufficiently inert to reactive metals such as titanium and its alloys to avoid alpha case formation at the high temperatures encountered during diffusion bonding and superplastic forming. However, the use of yttria in its commercially available fine powder form often resulted in excessive breakthrough pressures and extended time periods to achieve breakthrough, with the attendant dangers and disadvantages noted above. Thus, in the above patent which discloses use of such yttria as stopoff compound, breakthrough pressures prior to superplastic forming are noted as ranging from 25 to 250 psi, and at such breakthrough pressures, time required to achieve breakthrough can require up to one or more hours. In terms of a pressure-time product, the fine commercial grade yttria gave undesirable breakthrough values usually in excess of 5000 psi-minutes.
Accordingly, one object of the invention is the provision of an improved procedure for producing SPF/DB structures.
Another object of the invention is the provision of a superplastic forming and diffusion bonding procedure for the above purpose, which provides a low breakthrough pressure-time product prior to superplastic forming, to permit forming to occur uniformly, avoid strain rates in excess of the superplastic range, and avoid excessive local thinning an/or rupture of the metal sheets.
A still further object is the provision in the above noted superplastic forming and diffusion bonding process of a stopoff compound for application to preselected areas of the metal sheets, which is inert to reactive metals such as titanium and its alloys, and which also provides low breakthrough pressure-time products to facilitate uniform and successful forming during the subsequent superplastic forming operation.